The invention relates to a fuel cell with a proton-conducting polymer electrolyte membrane.
Polymer-electrolyte fuel cells, such as are known from DE-A1-195 42 475 (U.S. Pat. No. 5, 928,807), are equipped with semi-permeable membranes, which separate an anode compartment and a cathode compartment of the fuel cell from one another, but are designed to enable the transport of protons from the anode to the cathode.
From the contribution made by Wang et al. xe2x80x9cA H2/O2 fuel cell using acid doped polybenzimidazole as polymer electrolytexe2x80x9d in Electrochimica Acta, GB, Vol. 41, No. 2, of Feb. 1, 1996, pages 193-197, but also from the generic U.S. Pat. No. 5, 525,436, is known a thermoplastic polymer, which is used as the membrane in a PEM fuel cell that is operated at more than 120xc2x0 C. The polymer is phosphoric acid-doped polybenzimidazole. FUEL CELL WITH POLYMER ELECTROLYTE MEMBRANE
One customary membrane is a fluoropolymer, e.g. Nafion(trademark), which remains stable at the fuel cell operating temperatures of approximately 80xc2x0 C.-100xc2x0 C., and which is characterized by high ionic conductivity for protons.
In addition to the high cost of this type of membrane, a further problem arises with fuel cells of this type if pure media, above all pure hydrogen, are not used, and instead media containing hydrocarbons are used, in which the hydrogen must first be separated out to allow operation of the fuel cell. This causes the hydrogen to reach the anode compartment contaminated with carbon monoxide and carbon dioxide. While the presence of carbon dioxide is not critical, already small quantities of carbon monoxide will cause catalysts in the fuel cell system, such as platinum, that come into contact with the pollutants to become contaminated.
This problem is further intensified in that, in order to reclaim unused hydrogen, the exhaust gas from the reaction is normally fed at least partially into the circuit, which allows the carbon monoxide content to become concentrated.
In order to prevent this undesirable contamination of the catalysts, costly purification measures must be performed to keep the carbon monoxide concentration low enough, preferably far below 0.5% by volume. The necessary gas purification steps are expensive, and tend to reduce the degree of effectiveness of the fuel cell system.
An object of the invention is to provide a fuel cell, for the operation of which a carbon monoxide concentration of 2% by volume can be achieved, without requiring additional cleaning measures.
The object is attained according to preferred embodiments of the fuel cell according to the present invention.
A fuel cell as specified in the invention is comprised of a membrane made of a thermoplastic polymer, which has a permanent service temperature of at least 100xc2x0 C. The advantage of using this polymer is that it allows the temperature level of the arrangement as a whole to be raised. The result is that the so-called Boudouard equilibrium, which determines the ratio of carbon monoxide, carbon dioxide, and carbon in a reformate of a medium containing hydrocarbons, is shifted in favor of the non-critical carbon dioxide and to the disadvantage of the harmful carbon monoxide; in other words, less carbon monoxide forms in a gas purification stage that is connected to the fuel cell, and/or prior to entry into the anode compartment.
At the same time, a further consequence is that the anode catalyst, preferably platinum, adsorbs considerably less carbon monoxide, and thus can tolerate higher carbon monoxide concentrations in the gas flow.
The fuel cell is most preferably operated at a permanent service temperature of at least 120xc2x0 C.
Another advantage of the high temperature level of the fuel cell is that the expenditure required for cooling the reaction gas, which normally exhibits temperatures of approximately 300xc2x0 C. upon exiting the gas purification system at the inlet to the fuel cell, is reduced considerably. Substantial improvements in the degree of effectiveness of the fuel cell arrangement are made possible by saving money on gas purification and on the cooling system.
One preferred membrane is comprised of one or more polymers and/or copolymers and/or polymer blends of polysulphone (PSU), polyether sulphone (PES), polyether etherketone (PEEK), polyimide (PI), and polybenzimidazole (PBI).
In addition, the membrane is preferably made of an asymmetrical polymer that is permeated with pores extending from one flat surface of the membrane to the other flat surface. One of the flat surfaces of the membrane is also provided with a continuous coating layer made of the same polymer, so that the membrane area that is permeated with the pores forms a support structure for the coating layer. The pore diameter may be constant along its longitudinal axis, or may increase from one flat surface to the other; preferably the pores having a small diameter extend away from the coating layer.
The advantage of the coating layer that is protected by the protective structure is that the continuous coating layer allows a higher pressure gradient to be maintained between the anode compartment and the cathode compartment, with simultaneous favorable proton conductivity.
The membrane is most preferably modified, in order to increase its ionic conductivity. The membrane is preferably bombarded with electromagnetic and/or particle radiation, so that radiation-induced defect points will contribute to proton conductivity. A further preferred modification consists in providing the thermoplastic polymer with agents, for example, functional groups, which increase the proton conductivity.
In addition to increasing the effectiveness of the system via lower gas purification and gas cooling requirements, the fuel cell specified in the invention also enables an increase in the power density of the fuel cell as a result of the increased service temperature and/or the increased operating pressure.